Quote=kk7cw:
The "nanowatts" you mention is the amount of RF energy radiated by the antenna conductor. The power level I mention is the transmitter final PA input power. They are different; quite different. The use of a lossy antenna would allow 10 to 25 milliwatts of transmitter input power to produce the appropriate power for legal field strength.
Understood. But even if the PA output power of a Part 15 FM tx is only 10% of the PA input power, that 10% (1-2.5 mW in your example) is still far more power than can be radiated even by a low-gain antenna without exceeding the Part 15 FM limit at 3 meters.
Even a very short "whip" antenna is a very efficient FM radiator. In fact the peak gain of a 1/10-wave dipole is about 91.5% that of a 1/2-wave dipole -- and well above that of an isotropic radiator.
But suppose the whip antenna radiated only 10% of the 1 mW output of a Part 15 FM tx. The peak field at 3 meters then would be 23.4 mV/m, or about 94X higher than the Part 15 limit.
FCC certification does NOT ensure legal operation either. Just ask the folks at Ramsey Electronics.
It will if the certification was properly done, and the tx+antenna system is operated in the same configuration as when they were certified together in a single test. But of course it will NOT meet Part 15 FM rules when in operation if the user changes the system configuration (uses a different antenna design or adjustment, adds an r-f amplifier, etc). In such cases the original certification is invalid, anyway, and the responsibility for compliant operation must be assumed by the operator.
If you talk with Keith Hamilton (one of the good guys) about violating the letter of the rules with a Rangemaster transmitter, he will tell you they can be operated outside the rules and regs.
Certainly, but not without risk. I wonder if this statement you attribute to him really is his official position before the FCC. I doubt it. More likely it is just "wishful thinking," useful to those wanting to operate that way.
Regarding the isotropic antenna. ..But theoretical gain, at least in popular publications on the subject, puts the gain figure of a dipole at 2.26 db gain over that of an isotropic antenna. That would make the gain of a 1/4 wave and the theoretical isotropic very close to one another, with the 1/4 wave system winning out due to non-linearity in the radiation pattern. AM broadcast rules have used this thumbnail comparison for over 50 years. Your text books may not use the same figures or definitions, but broadcast engineering practice for the past half century certainly has.
The 1/4-wave vertical antenna and radial ground system used in AM broadcasting have very nearly the gain of a 1/2-wave dipole (which is 2.15 dBi, not 2.26 dBi), not that of an isotropic radiator. Please refer to virtually any antenna engineering textbook to confirm (Kraus, Balanis, Terman etc).
The fundamental question remains how do uneducated everyday hobbyists stay comfortably inside the boundaries of the current Part 15 rules and regulations? I have asked this question on several boards and groups, and as yet no one has been able to offer a satisfactory fool proof solution or answer.
Actually, I did answer that question earlier in this thread, and again in this post. The answer is "Buy a Part 15 FM tx and antenna that have been certified together as a compliant system, and use it in that configuration."
These posts are getting quite technical, so maybe we should take it off board to avoid the clutter? You have my email address.
Rgds.
Neil,
My apologies for any misunderstanding on my part. I should write at some other time other than late at night. Your final paragraph is the most constructive analysis of real world testing anyone has offered in some time. Thank you for your patience.
My point is exactly this, if a hobby broadcaster were to build a micro power FM transmitter kit and use a very short lossy antenna, it is still possible under the "letter" of the rules to be in violation. Hence, the opportunity for capricious and uneven enforcement of the rules by inspectors. And when the penalty is $10,000 dollars per violation, maybe the rules could be more clear and consistent.
The current rules are like a job description suggesting you should show up for work early in the morning. And then further clarifying, that would be anytime after 4:30 AM but before 10 AM, but definitely not after 10 AM or before 4:30 AM. The work day will end at 5 PM. What time is more correct than any other? When would you show up for work? None of the parameters are incorrect, just non-definitive and not very constructive. The chance of getting the boss ticked-off is pretty good. It would take at least one wrong time showing up to work to know for sure. I hope I have made my point.
But, once again let me congratulate you on GETTING IT. By being clear about what the primary focus of the rules are, you advance the service of hobby/Part 15 broadcasting. I count it a priviledge to be able to discuss this with you on this forum.
Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org
Rich,
I really do appreciate your diligence in answering these questions. You are correct the technical level of these latter discussions has gone beyond the useful scope of the forum. However, by focusing on the primary focus and intent of the FCC rules for hobby broadcasting, we help those who are learning expand their knowledge. Using simple real world scenarios are the most helpful. If you teach electronics, you know catching the interest of the student and developing a passion for learning more is what it is all about.
Keep us on the straight and narrow path, and continue to share your treasure trove of knowledge. It lights the path for all of us; even those of us who have already developed a passion for radio.
Thank you for your contribution to the cause.
Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org
Quote=kk7cw
...Keep us on the straight and narrow path, and continue to share your treasure trove of knowledge...
Thanks for the good words. However my goal in posting here is not to keep people on the straight and narrow--only to help them recognize and understand what that consists of.
Whatever readers do with that information is up to them. But at least they can, if they wish, make their operating choices based more on engineering reality, and less on speculation.
Hi Marshall,
No apology needed and thanks for your supportive words.
I have enjoyed and learned from our dialogue and will continue to do so.
Neil
Hi,
I ran MLR (Manteca Local Radio) on a bench-built transmitter (Not a kit, but a homebrew). My radiator was probably a couple of inches over the limit (to get the exact match required) - but no more than 3 inches.
I used the base loaded design.
My ground was from the tx to the mast.
NOW, the mast was grounded to cold water pipe (if you have copper - DO THIS!!), plus ratshack grounding rods.
2 FCC agents live(d) in Manteca. I had spoken with one on a regular basis(I made the call, not the other way round). I told them where I was, what I was doing, why I was doing it.
The thing, I think, that made me "ok" was that I did not broadcast anything political - at all. They like that, cause then that makes you non-controversial.
The last guy you want to see...
The FCC has a link on their website under Enforcement Bureau, Field Issued Notices which is the results of all the busts, Part 15 and otherwise. This site is updated regularly.